Add RELEASE_BUILD_FLAGS_IN_PROTOBUF

This will guard the conversion to using protobufs for build flags.

To ease testing for those working on the migration, add a temporary
release config "protobuf_flags" which is === "trunk_staging", with
RELEASE_BUILD_FLAGS_IN_PROTOBUF=true.

Bug: 328495189
Test: manual
Change-Id: I7d40e5bb923cf84609e2f3a3cbd47c5a5d03a983
Merged-In: I7d40e5bb923cf84609e2f3a3cbd47c5a5d03a983